There is no law that requires ISPs to block P2P traffic or monitor the behavior of their users. In fact, the Digital Millennium Copyright Act provides what some have called a "safe harbor" for Internet Service Providers. The legislation protects ISPs from the behavior of their networks' users.
The disadvantages of packet shaping lie in two areas: customer relationships and costs. From a customer relationship perspective, packet shaping has the potential to anger clients unwilling to accept their ISP's decision regarding which traffic should and should not be rate-limited. In your question, you alluded to the recent allegations that Comcast blocked and rate-limited P2P traffic. This resulted in a national media frenzy that incurred a public relations cost likely so high that it exceeded any cost savings achieved by reducing bandwidth consumption. Additionally, packet shaping requires the addition of new technology to the infrastructure, increasing overhead and incurring costs related to equipment acquisition, policy maintenance, troubleshooting and monitoring.
This was first published in March 2008